As a conventional apparatus for collecting biomedical information such as a heart rate, a respiration rate and movements of the body, an apparatus wherein electrodes for detecting various kinds of information are attached to the human body and signals detected by the electrodes are transmitted to a measuring instrument over lead wires to collect biomedical information of the human body is used frequently.
In such a conventional apparatus as described above, since the electrodes for information detection are attached to the human body, they are liable to be displaced from their original positions during use, resulting in variation of the output signals thereof and the collection lead wires are liable to suffer from disconnection at crossing points of the electrodes or folds of bedclothes, and where commercial power supply is used, there is a danger that, if the collection lead wires should contact with the living organism, then the living organism may receive an electric shock. Further, a lead wire is very likely to act as an antenna and suffer from external electromagnetic wave noise. In this manner, the conventional apparatus has various subjects to be solved. Further, not only accurate information cannot be collected for a long period of time, but also the freedom of the human body is lost owing to electrode fixing instruments and lead wires such that the human body is restricted even from tossing and turning in bed.
As a countermeasure for solving the problems of conventional biomedical signal detection apparatus of the type described, an apparatus disclosed in the official gazette of Japanese Patent Laid-open No. Hei 10-14889 has been proposed.
This apparatus includes body movement measurement means for measuring an oscillation signal of a living organism based on a series connection electrostatic capacity of a first electrostatic capacity formed between a first electrode and the living organism and a second electrostatic capacity formed between a second electrode and the living organism, and body pressure measurement means for measuring a body pressure signal originating from the weight of the living organism by means of the first or second electrode and a third electrode, and further includes calculation means for calculating a characteristic amount of the living organism such as the body weight, a heart rate, a respiration rate, an active mass or a life condition based on outputs of the body movement measurement means and the body pressure measurement means without adhering a measurement electrode directly to the living organism.
However, the apparatus disclosed in the official gazette of Japanese Patent Laid-Open No. Hei 10-14889 uses an electrostatic capacity type sensor to detect an oscillation signal of a living organism and uses a pressure-sensitive element to detect a body pressure signal originating from the weight of the living organism.
Generally, the electrostatic capacity type sensor is not good in temperature characteristic and exhibits a variation of a signal in a low frequency region in the proximity of the direct current. Meanwhile, the pressure-sensitive sensor has a creep characteristic and so forth and is low in response speed. In short, the pressure-sensitive sensor is poor in measurement accuracy of the absolute pressure and cannot catch a dynamic high frequency signal. Although it is a possible idea to use a distortion resistive element as the pressure-sensitive sensor, the output signal of the distortion resistive element is greatly dependent on environment such as installation conditions or the temperature. As a result, conventional biomedical signal sensors have a subject to be solved in that they are subject to such a restriction that the user itself performs zero point adjustment or gain adjustment every time a measurement is started, or a protection apparatus for stabilizing the installation environment of the sensor is provided separately, or else the biomedical signal sensor is used only as an on/off switch.
As a method which makes up for the drawback described above, a method has been proposed wherein a flexible closed body such as an air bag is placed under a living organism and a pressure variation in the closed body by a movement of the living organism is measured by a pressure sensor or a microphone to collect biomedical information.
However, in order to use this method to collect biomedical information in full and over a wide range, the closed body must be laid over a wide area.
Further, it is necessary to keep the internal pressure of the closed body higher than the external air pressure and use the closed body in an expanded condition.